1. Field of the Invention
The present invention relates to an electronic endoscope system that is capable of identifying an abnormal tissue, such as cancer in an organ, using auto-fluorescence emitted from the tissue onto which the excitation light is illuminated.
2. Description of the Related Art
A tissue in the organ, which the excitation light having a wavelength in the ultraviolet range or in that vicinity is illuminated onto, enters into an excited state so as to emit auto-fluorescence. The intensity of the auto-fluorescence from abnormal tissue (for example, cancer) is weaker than that of normal tissue. Therefore, it is known that the auto-fluorescent endoscope system identifies the abnormal tissue, based on the auto-fluorescent image that is formed from the auto-fluorescence.
Incidentally, the lumen (interior hollow area) in the organ does not emit the auto-fluorescence, so the intensity of the auto-fluorescence in the lumen is weak, similarly to with the abnormal tissue. Thus, the abnormal tissue is not distinguished from the lumen by only the auto-fluorescent image. Accordingly, in the auto fluorescent endoscope system, the normal image that is obtained when the white light is illuminated onto the tissue is also referred to in order to identify the abnormal tissue.
The lumen is dark in the normal image because it cannot reflect white light, but the abnormal tissue is bright in the normal image because it can reflect white light. Therefore, the part that is dark in the fluorescent image but that is bright in the normal image is identified as the abnormal tissue.
Conventionally, the normal image and the fluorescent image are arranged right to left on the same monitor, so that in a user can identify the abnormal tissue optically using both images simultaneously displaying on the monitor. However, with this method, it is difficult to identify the abnormal tissue because the image displaying area for each image is small. Further, this method requires that the identification of the abnormal tissue depend on the doctor's divination and experience. Therefore, there is a fear of a doctor with insufficient experience missing out on identifying the abnormal tissue.
Therefore, recently improved auto-fluorescent endoscope systems have been developed, as described below. For example, the abnormal tissue is automatically identified based on both the luminance values of the normal image and the luminance values of the fluorescent image, and the pseudo-color (yellow or red, for example) overlaps onto the area corresponding to the abnormal tissue, as shown in Japanese Unexamined Patent Publication (KOKAI) No. 2003-290130. Due to this method, it is easy to identify the abnormal tissue even if the doctor does not have much experience.
However, it is impossible to observe the tissue itself corresponding to the area where the pseudo-color overlaps with this method. Further, if a part of the tissue bleeds, the bleeding part may be misidentified as abnormal tissue, so that the pseudo-color overlaps onto the bleeding part, because the luminance value of the bleeding part can be low in the fluorescent image but can be high in the normal image, similarly to with the abnormal tissue.
As shown in Japanese Unexamined Patent Publication (KOKAI) No. 2003-126014, the fluorescent image signals corresponding to the fluorescent image are generated by illuminating the excitation light onto the tissue, and two kinds of normal image signals are obtained when the different color lights are illuminated onto the tissue, respectively. A pseudo-color display image is generated based on two kind of normal image signals and the fluorescent image signals, so as to identify the normal tissue by the first pseudo-color and the abnormal tissue by the second pseudo-color. However, the pseudo-color display image does not express the tissue by its natural color; therefore, it interferes with correct observation of the tissue.